This invention relates to a fuel injection pumping apparatus for supplying fuel to an internal combustion engine and comprising a housing, a rotary distributor member mounted in the housing and arranged to be driven in use, in synchronism with the associated engine, a bore formed in the distributor member and a pumping plunger therein, a delivery passage in the distributor member and a plurality of outlet ports in the housing, said outlet ports in use being connected to the injection nozzles respectively of the associated engine, said outlet ports being positioned such that the delivery passage registers in turn therewith as the distributor member rotates, inlet port means in the housing, a source of fuel under pressure connected to the inlet port means, inlet passage means in the distributor member, said inlet passage means being connected to said bore, said inlet port means and said inlet passage means being arranged to allow fuel flow to said bore during at least part of the time said delivery passage is out of register with an outlet port, and cam means including a plurality of cam lobes for imparting inward movement to the plunger during part of the time the delivery passage is in register with an outlet port.
Such apparatus is well known in the art however, one problem is that the amount of fuel delivered through each outlet port can vary even though the amount of fuel supplied to the bore at each filling stroke of the apparatus is the same. This irregularity of delivery is thought to be due to pressure waves which travel up and down the pipelines connecting the outlet ports with the injection nozzles. When during the delivery stroke the crest of the cam lobe is reached, the supply of fuel to a particular nozzle ceases. The plunger is allowed to move outwardly a small amount and is then held against movement. The purpose of the outward movement is to reduce the pressure in the pipeline and at the nozzle and this allows a valve member in the nozzle to close quickly to prevent dribble of fuel from the nozzle orifices. The closure of the valve member in the nozzle results in a pressure wave which travels back towards the outlet port and into the various passages in the distributor member. The wave is reflected and travels back towards the nozzle and may be re-reflected many times. The purpose of the dwell on the trailing flank of the cam lobe is to ensure that a predetermined volume of fuel is relieved from the pipeline however, the pressure wave may not have become attenuated by the time the delivery passage moves out of register with the outlet port and in this situation the pressure in the various passages in the distributor member may be low to the extent that there could be a cavity in the fuel. Assuming that equal volumes of fuel are supplied to the bore at each filling stroke, it is clear that during the following delivery stroke less fuel will be delivered.
It is known to provide in each outlet port a so-called pressurising valve and the duty of these valves is to try to ensure that the pressure of fuel in the various passages in the distributor member at the instants at which the delivery passage moves out of register with the outlets, is substantially the same. Such valves are however expensive since they have to be carefully made to ensure that their operating characteristics are substantially the same.